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# Copyright (c) 2008-2017 MetPy Developers. |
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# Distributed under the terms of the BSD 3-Clause License. |
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# SPDX-License-Identifier: BSD-3-Clause |
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"""Contains calculation of various derived indices.""" |
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import numpy as np |
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from .thermo import mixing_ratio, saturation_vapor_pressure |
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from .tools import get_layer |
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from ..constants import g, rho_l |
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from ..package_tools import Exporter |
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from ..units import check_units, units |
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exporter = Exporter(globals()) |
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@exporter.export |
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@check_units('[temperature]', '[pressure]', '[pressure]') |
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def precipitable_water(dewpt, pressure, top=400 * units('hPa')): |
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r"""Calculate precipitable water through the depth of a sounding. |
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Default layer depth is sfc-400 hPa. Formula used is: |
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.. math:: \frac{1}{pg} \int\limits_0^d x \,dp |
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from [Tsonis2008]_, p. 170. |
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Parameters |
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---------- |
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dewpt : `pint.Quantity` |
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Atmospheric dewpoint profile |
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pressure : `pint.Quantity` |
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Atmospheric pressure profile |
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top: `pint.Quantity`, optional |
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The top of the layer, specified in pressure. Defaults to 400 hPa. |
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Returns |
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------- |
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`pint.Quantity` |
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The precipitable water in the layer |
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""" |
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sort_inds = np.argsort(pressure[::-1]) |
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pressure = pressure[sort_inds] |
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dewpt = dewpt[sort_inds] |
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pres_layer, dewpt_layer = get_layer(pressure, dewpt, depth=pressure[0] - top) |
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w = mixing_ratio(saturation_vapor_pressure(dewpt_layer), pres_layer) |
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# Since pressure is in decreasing order, pw will be the negative of what we want. |
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# Thus the *-1 |
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pw = -1. * (np.trapz(w.magnitude, pres_layer.magnitude) * (w.units * pres_layer.units) / |
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(g * rho_l)) |
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return pw.to('millimeters') |
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@exporter.export |
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@check_units('[pressure]') |
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def mean_pressure_weighted(pressure, *args, **kwargs): |
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r"""Calculate pressure-weighted mean of an arbitrary variable through a layer. |
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Layer top and bottom specified in height or pressure. |
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Parameters |
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---------- |
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pressure : `pint.Quantity` |
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Atmospheric pressure profile |
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*args : `pint.Quantity` |
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Parameters for which the pressure-weighted mean is to be calculated. |
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heights : `pint.Quantity`, optional |
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Heights from sounding |
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bottom: `pint.Quantity`, optional |
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The bottom of the layer in either the provided height coordinate |
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or in pressure. Don't provide in meters AGL unless the provided |
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height coordinate is meters AGL. Default is the first observation, |
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assumed to be the surface. |
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depth: `pint.Quantity`, optional |
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The depth of the layer in meters or hPa. |
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Returns |
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------- |
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`pint.Quantity` |
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u_mean: u-component of layer mean wind. |
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`pint.Quantity` |
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v_mean: v-component of layer mean wind. |
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""" |
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heights = kwargs.pop('heights', None) |
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bottom = kwargs.pop('bottom', None) |
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depth = kwargs.pop('depth', None) |
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ret = [] # Returned variable means in layer |
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for datavar in args: |
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layer_p, layer_arg = get_layer(pressure, datavar, heights=heights, |
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bottom=bottom, depth=depth) |
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# Taking the integral of the weights (pressure) to feed into the weighting |
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# function. Said integral works out to this function: |
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pres_int = 0.5 * (layer_p[-1].magnitude**2 - layer_p[0].magnitude**2) |
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arg_mean = (np.trapz(layer_arg * layer_p, x=layer_p) / pres_int) |
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ret.append(arg_mean * datavar.units) |
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return ret |
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Unidata /
MetPy
